Three‐dimensional Vp and Vp/Vs structure at Loma Prieta, California, from local earthquake tomography

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Abstract
P and S‐P times from local earthquakes and P times from explosions at seismic stations in the vicinity of the 1989 Loma Prieta earthquake are inverted simultaneously to produce three‐dimensional images of Vp and Vp/Vs structure along the rupture zone. The results confirm the presence of the major heterogeneities identified in previous studies using P waves only, including high Vp along the rupture in areas of significant slip and a low‐velocity wedge along the San Andreas at the SE edge of the rupture. A change in rock properties evidenced by a significant Vp/Vs increase (equal to a 30% Poisson's ratio increase) from the middle crust to the upper crust may have been responsible for the upward termination of the main shock rupture. We speculate that the deep high‐Vp body that lies along the SE portion of the rupture zone consists of two bodies, based on model results indicating that the SW part of the body has relatively high Vp/Vs while the NE part has relatively low Vp/Vs.